This invention relates to apparatus and procedure for the belt casting of metal, more specifically the continuous casting of metal between endless belts, in the form of strip. In a notably important sense, the invention is concerned with methods and machines for casting metals such as aluminum (including aluminum alloys) and zinc, and other metals, which melt at moderate or low temperatures, between a pair of moving surfaces conveniently constituted of flexible heat-conducting bands or belts that have conventionally been metal belts in twin-belt casters of this sort.
Although the continuous casting of metal, including especially the casting of aluminum and similar light metals, in belt-casting apparatus has been under development for many years, and although useful improvements in guiding, stabilizing, and cooling the belts of such casters have been made, it does not appear that a number of important problems of control as to a desired intimacy of contact between the belts and the solidifying metal and as to other characteristics of stabilizing and cooling the belt, have been fully recognized or that means for providing such control in an effective manner have been heretofore known or available in prior apparatus.
Extended investigation has revealed that continuing, intimate contact between the belts and the metal, while maintaining thorough cooling of the reverse surface of the belt, is necessary, in continuous casting, for truly satisfactory results and a desirably high production rate of accurately dimensioned strip having good surface and internal properties. In particular, it has been found that arrangements and procedure for maintaining the described intimate contact between the belts and the metal, and for effectuating control for that purpose, should recognize differences in condition along the path of metal through the casting zone, including highly localized variations.
With these requirements in mind, the present invention is aimed at achieving a uniform and high cooling rate while compacting the solidifying metal and while maintaining satisfactory cross-section profile, e.g. a desired uniformity of shape of the cast strip, including uniformity of gauge and as close to a flat, plane surface on both sides as possible. The nature of solidification of the metal, both at successive general zones along the path and at places where localized irregularities may occur, needs to be considered in control of the process. There should be attention to the amount of metal in fluid state at any zone or place, the effect of its metallostatic head on the belt (either directly or through a shell), and the nature of the solidifying shell at each face of the mold; i.e. whether a weak agglomerate or particles, a solid but flexible shell, or a rigid shell becoming integrated with the shell on the other face.
In some prior apparatus, there have been arrangements where the guiding or support means for each belt along the casting path, such as a set of rollers or the like spaced along and across the path, have been given some yieldability as a complete, unified assembly, or where a transverse row of such rollers or elements across the belt path has been given a resilient mounting which tends to push such unitary set of rollers (as a whole) continuously against the belt and the belt correspondingly against the metal, but these proposals have failed to appreciate the localization, by zones or as to small, random places, of the problem of contact between belt and metal. Experience has indicated that trouble due to a local separation, even very slight, between the solidifying shell of the metal and the adjacent belt surface, can become progressively severe, in the sense that with such failure of contact, the normal heat transfer and temperature conditions at the surfaces of the metal and belt and through the belt are changed in this very local spot, with the likelihood of resulting thermal distortion of the metal shell and/or belt, that may in turn create further separation at adjacent localities, and still further thermal effects.
The foregoing difficulties can become particularly troublesome in attempts to increase the speed of continuous casting, or to cast relatively thin strip with high accuracy, or to cast an alloy where solidification occurs over a range of temperature, with corresponding delay in reaching a frozen shell of fully solid strength. An important object of the invention is therefore to provide improved methods and apparatus for continuous casting, wherein superior contact is maintained with the metal, and with correspondingly superior cooling and superior control of the dimensions of the product.
Other areas wherein the objects of the invention aim for improvement in belt casting apparatus and procedure are with respect to: the general organization of the equipment particularly for mounting and retaining the belts in exact position as desired; the efficiency of handling and utilizing liquid coolant for the belts; and instrumentalities for controlling the transverse profile of a belt at the mold space, so as to achieve desired, uniform, dimensional accuracy of the cast strip. As stated, despite useful recent advances, particular problems have nevertheless been found to reside in the control of the process for maintenance of continuous, adequate cooling and positional stabilization of the belts, against undesired shape or internal defect of the casting, or uneven surface or breakouts, a special need being to take account of varying requirements along different zones of the casting path, as well as varying difficulties that may occur in highly localized ways.